The Security Service Act 1989 (c. 5) is an Act of the United Kingdom Parliament. The Act established a statutory basis of the UK Security Service (MI5) for the first time. Prior to the Act, successive UK governments had denied the existence of MI5, despite its operation since 1909, . The Act begins, "There shall continue to be a Security Service .."

The first section defines the function of the Service as

the protection of national security and, in particular, its protection against threats from espionage, terrorism and sabotage, from the activities of agents of foreign powers and from actions intended to overthrow or undermine parliamentary democracy by political, industrial or violent means.

In the next paragraph it adds the further function, "to safeguard the economic well-being of the United Kingdom against threats posed by the actions or intentions of persons outside the British Islands."

The Act was amended by the Security Service Act 1996 to include supporting the police and other law enforcement agencies in the prevention and detection of serious crime.

• David Maxwell Fyfe, 1st Earl of Kilmuir
• Spycatcher